Production of minute apertures in thin films



United States Patent M 3,494,810 PRODUCTION OF MINUTE APERTURES IN THIN FILMS Raymond E. Skoda, Scofia, N.Y., assignor to General Electric Company, a corporation of New York Filed Dec. 9, 1966, Ser. No. 600,568 Int. Cl. B29C 17/08; B29g 7/02 US. Cl. 156-13 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of forming minute apertures in a metallic film and in particular to the utilization of an emulsion of water and a resinous solution to form, upon drying, an apertured shield overlying the metallic film to control the subsequent etching of the film.

In switching the storage site of a superconducting continuous film memory, current is passed simultaneously through two intersecting drive lines situated above a memory plane. and the site underlying the intersection of the two drive lines is switched. All other sites underlying the activated drive lines are half excited during the switching process and tend to suffer some flux switching which, after many half excitations, may lead to degradation of the information stored at the half excited sites so that they can no longer be sensed. In order to reduce the information degradation at the half excited sites, films forming the memory plane have been deposited upon substrates preheated to a temperature suflicient to produce apertures in the deposited film. Because the apertured planes formed by control of the substrate temperature during deposition are necessarily very thin, the memory planes often are unstable on temperature cycling. Furthermore, because the pattern and size of the apertures are non-uniform and generally uncontrollable, the electrical switching characteristics of the cells forming the plane often are diverse.

It is therefore an object of this invention to provide a method of forming minute apertures in thin films of various thicknesses by etching the film.

It is a further object of this invention to provide a method of producing an apertured memory plane wherein the size of the apertures is controllable.

These and other objects of this invention generally are accomplished by dispersing water droplets throughout a liquid resinous solution, e.g. a soluble resin such as polyolefin, polyproplyene, polystyrene, or polyethylene dissolved in a solvent, to form an emulsion and overlying the emulsion upon the film to be perforated. The emulsion subsequently is dried to evaporate the water droplets and the residual solvents of the resinous solution thereby forming a hardened resinous shield having perforations at the location of the water droplets. Acid then is applied over the hardened resinous shield to etch the underlying film. Because the resinous coating must tackify prior to the evaporation of the water droplets forming the inner phase of the emulsion in order to form apertures at the location of the water droplets, the resin thinner employed as the solvent must evaporate before or simultaneously withthe water droplets. Apertures approximately 1 mi 3,494,810 Patented Feb. 10, 1970 cron in diameter can be obtained when the dispersion of the water droplets is accomplished by the introduction of water into the resinous solution and the subsequent vigorous agitation of the immiscible liquids such as may be accomplished in an ultrasonic vibrator.

Convenient sources of the liquid resinous solution used in this invention are commercial photoresists which are sold under diverse trade names dependent upon the utilization intended. Many commercial photoresists including KMER consist essentially of a polyolefin resin dissolved in xylene along a small amount of sensitizer. The sensitizer acts as an electron acceptor-donor when irradiated with ultraviolet light and functions in negative type photoresists to activate the cross-linking of the polyolefin resin. Among the sensitizers utilized in various photoresists are cyano, amino, nitro, azo compounds or ketones, quinones, and antrones, which are added in quantities of of 0.1 to 1.0 mole per cent. Although the main function of the sensitizers employed in photoresists generally is to control polymerization of the resin to the extent that the resultant molecules no longer dissolve in the developer solvent, some sensitizers also are activated during the bake cycle to further cross-link the resin thereby aiding in the adherence and chemical resistance of the photoresist. Therefore, although ultraviolet light is not utilized to produce the desired pattern in the photoresist material in the method of this invention, a negative type sensitizer, e.g. 2,6-bis (p-azido benzyledene)-4-methycyclohexanone utilized in KMER photoresists, can assist in the adherence and chemical resistance of the resist. Positive type sensitizers similarly function to assist polymerization as well as to enhance the adherence and chemical resistance of the resinous shield.

Because the size of the water droplets forming an emulsion with the resinous liquid is determined by the amount of agitation of the immiscible fluids, the size of the apertures within the resinous shield formed by the water droplets upon drying of the emulsion can be controlled. Similarly, the ratio of Water to liquid resin will control the density of the apertures both in the resinous shield and the etched film utilizing the perforated shield. By utilizing both a high ratio of water to resinous solution and ultrasonic vibration to emulsify the liquids, the hole density can be made very high, e.g. the hole-to-hole distance will be small compared to the cell size of the memory plane. The cell characteristics of a film, being a function of the ratio of film area to aperture area, will depend more upon the quantity of holes than on the individual dimension of a single aperture when the etching shield utilized in perforating the film is formed from a vigorously agitated emulsion having a relatively high percentage of water. Thus an exact formity in the size of the apertures is not critical when the hole density per cell is high.

The features of this invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to the organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a flow diagram of the method of this inven tion for producing minute apertures in thin films, and

FIG. 2 is an enlarged viewing of a thin film apertured by the method of this invention.

The method of this invention, as portrayed in FIG. 1, generally includes the mixing of water and a resinous solution, e.g. a polyolefin dissolved in a solvent, such as xylene, to form an emulsion which is applied over a clean metallic film. The resinous emulsion then is dried to provide a hardened shield having perforations at the 10- cations of the water droplets for the subsequent etching of the film. Because the resin must tackify before the evaporation of the suspended water droplets to prevent the voids from being filled by the resin, the solvent employed either must have a lower evaporation temperature than water or must unite with the water to evaporate simultaneously.

The emulsion used to form the perforated shield for the etching of the substrate is prepared by introducing a quantity of water into a composition of resin dissolved in a thinner with the ratio of the Water to the resinous solution being determined by the desired ratio of metal to apertures in the perforated film. The most convenient sources of the resinous solution suitable for use in this invention are commercial photoresists which are marketed under a variety of trade names, e.g. KOR, KPR 2, KMER, KTFR, TPR, sold by Eastman Kodak, Rochester, N.Y., and AZ 1350, sold by Shipley, Wellesley, Mass. The method of this invention is operable in varying degrees of effectiveness with all the above mentioned photoresists, e.g. KMER and KTFR form very viscous emulsions with a tendency for rapid separation of the immiscible liquids while the AZ 1350, and KOR emulsions are relatively fluent and remain emulsified for a relatively long period of time. This is notwithstanding the fact that some of the photoresists are negative acting, that is, the sensitizer upon activation by ultraviolet light reacts with the polyolefin resin to promote polymerization of the resin to the extent that the resultant molecules no longer are dissolvable in the developer solvent, and some are positive acting, e.g. the exposed portion of the resin remains unpolymerized and can be washed away.

The resinous solution and the water are agitated either ,manually or mechanically to provide an emulsion with the water being the internal phase. The size of the water droplets forming the emulsion will generally be directly dependent upon the intensity, of agitation, with droplets having a diameter of 1 micron being obtainable through the utilization of an ultrasonic vibrator. Because the water-resin emulsion will tend to separate upon standing for a prolonged period, the coating of the substrate should occuras soon as possible after formation of the emulsion.

After the substrate has been cleaned, the emulsion is applied to the substrate by any of the commonly utilized methods, e.g. dipping, spraying, or whirl coating. The coating thickness should be of a dimension smaller than the diameter of the water droplets formin the emulsion in order to permit an aperture to form at the location of the water droplets upon subsequent drying of the coating. Thus, for apertures having a diameter of approximately 1 micron, the thickness of the emulsion coating upon the substrate preferably should be approximately 0.5 to 0.8 micron.

After coating, the liquid emulsion is dried to completely eliminate both the suspended water droplets and the solvent employed to dilute and liquefy the resin. The drying preferably is done in an oven which permits heating of the coated substrate by infrared radiation from both sides. Thus the solvent evaporates from the interface between the film and the substrate towards the still gelatinous surface of the layer which can be easily permeated. The application of heat only from the top of the coated substrate to dry the emulsion is not preferred because a crust of dry resin will form atop the emulsion and a portion of the solvent can become trapped intermediate the crust and the substrate thereby reducing both the adhesion of the film to the substrate and the chemical resistance of the coating. During the application of heat, the solvents having the highest volatility (highest vapor pressure) evaporate first gradually leavin behind any solvents with lower vapor pressures. Enough solvent must evaporate before the complete evaporation of the water droplets that the resin will tackify to insure the preservation of the apertures formed by the water evaporation. Continued baking of the film results in a solidifi? cation of the resin produced by an expulsion of the solvents. Because the resinous coating is not exposed to light in the performance of the method of this invention, the particular sensitizer utilized in the photoresist is not of extreme importance. Furthermore, it is possible to utilize the method of this invention with a resin having no sensitizer at all. However, because some sensitizers are activated during drying of the emulsified coating to facilitate cross-linking of polyolefin resins during polymerization, the adherence and chemical resistance of deposited polyolefin films are increased by the sensitizer. Heating is continued until expulsion of the solvent and water from the resin produces a relatively dry, acid impervious coating for the film. In the formation of the perforated shield, any non-water miscible resinous solution which tackifies at a temperature below the evaporation point of water to a degree that the resin will not flow to fill the voids remaining after the expulsion of the suspended water droplets is suitable for employment in this invention. Absolute resistivity of the dried resin to acid is not required because of the rapid speed of the etching process employed to perforate the film.

Acid then is placed upon the porous dried resinous shield to etch the underlyin plate in a conventional manner. After etching the solidified resin is removed by an organic solvent.

A film formed by the method of this invention utilizing an ultrasonic vibration is shown in FIG. 2. The apertures produced in the film are approximately 1 micron in diameter and are separated by a distance varying from less than 10 microns to approximately 50 microns. By making the hole-to-hole distance small compared with the cell size of the continuous film memory storage plane, a minimum of alignment is required between the drive lines and the hole pattern. Furthermore, because each cell will involve a multiplicity of holes, cell switching characteristics will depend upon the quantity of holes Within the cell rather than the dimensions of any individual hole. The presence of the holes in the superconductive metallic film acts as obstacles to flux motion and degradation of the information stored at information sites due to half excitations is reduced.

While several examples of this invention have been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from this invention in its broader aspects; and therefore the appended claims are intended to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A method of forming minute apertures in a metallic film which comprises the dispersing of water droplets throughout a resinous solution to form an emulsion, said resinous solution including a resin dissolved in a solvent having an evaporation point below or simultaneous with the water additive, coating the film to be perforated with said emulsion, drying said emulsion to evaporate said water droplets and the residual solvents of the resinous material thereby forrning a hardened resinous shield having perforations at the location of said water droplets, and applying acid over said hardened resinous material to etch the underlying film.

2. A method of forming minute apertures in a metallic film according to claim 1 wherein said dispersion of water droplets is accomplished by agitating a mixture of Water and a polyolefin resin.

3. A method of forming minute apertures in a metallic film according to claim 1 wherein said dispersion of water droplets is accomplished by introducing water into the resinous solution, and agitating the mixture of water and resinous solution in an ultrasonic vibrator.

4. A method of forming minute apertures in a metallic film which comprises vigorously agitating water in a solution of a resin dissolved in a solvent having an evaporation point below or simultaneous with the water additive thereby forming an emulsion having a multitude of Water droplets as the inner phase of said emulsion, the ratio of water to resinous solution being sufficiently high to provide an average spacing between droplets from 10-50 fold the droplet diameter, coating the film to be perforated with said emulsion, drying said emulsion to evaporate said water droplets and the residual solvents of the resinous material thereby forming a hardened resinous shield having perforations at the locations of said water droplets, 10

and applying acid over said hardened resinous material to etch the underlying film.

5. A method of forming minute apertures in a metallic film according to claim 4 wherein said emulsion is formed by agitation in an ultrasonic vibrator.

References Cited UNITED STATES PATENTS JACOB H. STEINBERG, Primary Examiner US. Cl. X.'R. 

